Cathodic protection of ships



Feb. 20, 1962 E. P. ANDERSON CATHODIC PROTECTION OF SHIPS 2 Sheets-Sheet 2 Filed June 30, 1958 INVENTOR.

EDWARD P. ANDERSON KM M 3M 5 cocoon oooooo United States Patent Ofiice 3,022,234 Patented Feb. 20, 1962 This invention relates to a system for cathodically protecting the propellers of ships and, more particularly, propellers made from steel. Furthermore, the invention provides a system which simultaneously protects the propellers and the hull of a ship against damage resulting from attack by sea water.

Heretofore, generally only bronze propellers have been used because this material resists to some degree corrosion and erosion, a combination customarily referred to as cavitation damage, and which includes pitting and loss of weight of the propeller. This invention, for the first time, permits the successful use on large ships of steel propellers which are far lighter and less expensive than bronze propellers.

Cathodic protection consists basically of applying an electrical current to an anode, immersed in an electrolyte,

and to the surface to be protected as the cathode, whereby the surface potential is maintained cathodic with respect to the electrolyte, thus preventing corrosion of the surface. When applying this principle of cathodic pro tection by impressed currents to painted ship hulls, the current density per square unit of painted surface has to be maintained below a predetermined value in order to prevent peeling off of the paint film. On the other hand, strong currents are necessary to protect unpainted propellers, especially steel propellers, and it was found that the required currents are a function or" the speed of rotation, i.e., more impressed current is needed to prevent cavitation damage to a rapidly rotating propeller than when the same propeller rotates slower or is at rest. It has been established by tests that efficient cathodic protection of a steel propeller is obtained by impressing a potential on the propeller which produces a current density of about 25 to 75 milliamperes per square foot of propeller surface when the speed of rotation is zero and 250 to 350 milliamperes per square foot of propeller surface when the speed of rotation is about 250 to 350 revolutions per minute.

The invention provides cathodic protection for the propellers and simultaneously for the hull of a ship by one system serving both purposes which, by automatic means, controls the output to each propeller of a ship and to the hull as well, whereby the power supplied to each component is automatically controlled and maintained at the optimum equired according to the prevailing conditions.

Generally speaking, the system for cathodically protecting the propeller of a ship comprises a power supply connected to the propeller and an anode connected to the power supply and immersed in the water, whereby the power is supplied to the propeller as a direct function of the speed of rotation of the propeller. This is suitably accomplished by means of a plurality of switches, 7

each of which is responsive to one distinct speed of rotation of the propeller and is so connected as to increase the power supplied to the anode above the power supplied at lower speeds of rotation.

Since the current impressed on the painted ship hull should not exceed the aforementioned optimum per' square unit of painted surface, and further, since this current is only a fraction of the current required for propeller protection, the system according'to the invention includes controls for varying the ratio between the power supplied to the propeller, in order to prevent damage to the paint film, suitably a control system which continuously senses the hull potential and automatically decreases the power supply when the hull potential exceeds a predetermined optimum. Such a control system is described in copending application Serial Number- 732,275 filed May 1, 1958, and an adaptation thereof suitable for the purpose of this invention is described below,

in order to impress a current on the propeller of a ship, a slip ring is mounted on the propeller shaft, and the cooperating brush is connected to the power supply output. It is therefore mandatory to insulate at least the shaft portion which transmits the current to the propeller and the stern bearings from the body of the ship. However, in view of the heavy current applied to the propeller and shaft, stray currents to the ship body and the hull have been found diflicult to avoid. Therefore, and to protect the hull pain against damage due to these stray currents, the control of the output as a function of the hull potential is almost a necessity with painted ship hulls.

For the sake of simplification, and in order to more clearly describe the invention, the following description is based upon a supply from a single-phase alternating current source. The actual system, as installed on large ships, is usually fed from the output of a three-phase generator but operates in accordance with the principles described herein; insofar as essential differences between the single-phase and the three-phase system exist, they will be mentioned hereinafter.

The invention will be further illustrated by reference to the accompanying drawing in which FIGURE 1 is a block diagram showing the relation ship between the components of one embodiment of the invention,

FIGURE 2 is a schematic circuit diagram of the cathodic protection system of FIGURE 1 when fed from a single-phase alternating current power supply, and

FIGURE 3 illustrates the operation of the two time output circuit, is decreased sufficiently to permit the hull potential to reassume the optimum. This is accomplished suitably by means of a reference half cell immersed in the Water and connected in a sensing circuit as described below. The power is subsequently fed to a rotation speed control circuit in which the control thereof as a function of the speed of rotation of the propeller is effected by means of a system of switches, each of which is responsive to a distinct rotation speed of the propeller and which is connected so as to increase the power supplied to the output circuit. Finally, the current in the output circuit, as controlled primarily as a function of v the hull potential and then as a function of the rotation speed of the propeller, is divided in the desired ratio between the propeller and the hull as the cathodes and, on the other hand, the anodes.

The control of the power as a function of the hull potential is accomplished by means of a reference half cell immersed in the water and cooperating with the hull to produce a current as a function of the condition of the hull. The reference half cell, which is suitably an assembly of silver chloride deposited on silver, is affixed to the hull in a protective housing so as to contact the water, but is otherwise insulated, and performs as one electrode of a voltaic cell; the other electrode thereof is the hull and both are in contact with the sea water. The

sensing circuit includes the reference half cell, an addi- 'justing the constant electromotive force in such a 'way that the two voltages are equal, but of opposite direc tion to the. point where the hull potential reaches the desired optimum, no current will flow through the sensing device.

- A more detailed schematic diagram of the cathodic protection system according to the invention is shown in FIGURE 2.

V The reference half cell 4, the hull, a constant source of electromotive force, suitably a dry-cell battery 6, a

variable resistor 8 and the main coil 10 of a galvanometer type relay 12 form the sensing circuit. 'A constant re- ,si'stor14is connected across the battery 6 and the resistor 8'is set in such away that the voltages across the resistor 14 are equal, but of opposite polarity when the hull potential is at an optimum. Thus, no current flows through the coil 10 of the sensing relayv 12. Upon any 7 change of the hull potential, one of the electromotive forces will prevail over the. other and a resulting, differential current will flow in one or theother direction,

causing the armature 16 of 'the sensing relay to deviate from the zero position.

'It has been found that a galvanometer type relay of" several milliamperes, is' a suitable device for the purpose of the invention, when provided with two contacts 18 and 20; at either side of the armature 16 and having, aresetting device. Such device consists of a coil .22 which is mechanically coupled to brackets. (not shown) adapted to return the armature .16 back to the zero position thereof, even when the main coil 10 is energized by a 9 microampere sensitivity, able to withstand current of 4 circuit from the main cable 42 over the conductor 69 at one side and the armature 16, the conductor 62', the time switch 30 and conductor 64, at the other side. It will be apparent from the foregoing that, when the time switch 30 closes the circuit, either the coil 56 or the coil 58 will be energized, as a function of the direction of current in the sensing circuit which causes the armature 16 of the sensing relay to close the circuit through the contact 18 or contact 20. In case the hull potential is at the predetermined optimum, the armature 16 remains in the zero position-and neither of the coils 56 or 58 is energized; as a result,.the armature48 of the switch 50 remains in the same position. i

'As illustrated in FIGURE 3, the cams 26 and 32 bperate injsuch a way that, during one complete cycle of about 1 to 3 minutes, both'contacts 24 and 30'are actuated to close their respective circuits intermittently, but each during the de-e nergized interval'of the other. Since the contact 24'ener gizes the resetting coil 22 of the sensing relay in the time interval a-b, no current flows through the contacts 18 and 20 when the resetting device operates since the contact 30 is open. During the interval b-c, the resetting device being tie-energized, the armature 16 is free to move in response to the current condition in the main coil 10 and assumes a position in contactwith either one of thecontacts 18 or 20, as a function of the direction of the resulting differential current in the sensing circuit, or remains in the zero position when no current'ilows through coil l0 because the hull potential is'at the optimum. During the time cd, the time switch. contact 30 is actuated to close; hence, switch coil 56 or 58will be energized as selected by the armature 16 of thesensing relay. Since the armature remains in the zero position when no current fiows through the coil 10, neither of the coils 56 nor 58 is energized in this case.

resulting ditferential current in the sensing circuit. The,

force of the resetting device being sufiicient to overcome the torque prodjuced by the current injthe coil 10 or an eventual sticking of the armature 16 to one of thecontacts 18 or 20, a maximum assurance of flawless performance ofthe sensing relay is guaranteed. The resetting coil 22 'isenergize'd periodically and intermittently. by means of a time switch 24 actuated by a ca m'26 which cam, in turn, is driven by. a motor 28. A second timeswitch 30 actuatedby a second'cam 32, suitably mounted on the ir ittently energize the adjustment of the control-circuit, as described below. i

The, signals emitted when the sensing relay armature 16' contacts either the contact point 18 or the contact point20 are used to control the power supplied to the.

' same shaft and driven by the motor 28, is used to interrotation speed control circuit. For this purpose, the

primaryjwinding 34 of a transformer 36 is connected to the power input38 over the feeding cables 40 and 42 and .the conductors 44 and 46, whereby the conductor 46 leads to the armature 48of 'a'snap action double throw switch 50,- thus formingthe input circuit of thesystem. The contact point 52 of the switch 50 is connected to one terminal of the primary 34, whereas the. other contact point 54' is connected to a tap of'the primary 34 such.

that the armature 48inserts the entire primary into the;

input circuit when contacting the point 52 and only a fraction thereof when contacting the'point 54. The

armature 48 ofthe switch 50 is actuated in one way. or the other by the coil 56 or 58, respectively, and retained in the selected position by means of a latch (not shown) or in any other suitable manner according to the 'construction of the switch. As a result, the switch 50 assumes one or the other position as a consequence .of' a current'impulse through either of the coils 56 or 58 and remains in this position, during any period of time, unless .the other coil is energized. By connecting the .coil to the contact 18 and the coil 58V to the contact 20,. one or the. othercoil is energizedby, connection into a a.

Referring to FIGURE 3 which shows the relationship between the portions of one cycle of the time switches, it has been found advantageous to operate under conditions. such that the relay contacts are without current at the moment of either opening or closing, Thus, at the time b, when the resetting coil releases the armature 16 to move towards the contact 18 or 20, the circuitthrough the selected contact and through the armature 16 is stillinterrupted by the switch contact 30. Only at the time c, when the relay armature 16 has already assumed one of the deflected positions, does contact 30 close the circuit. At the 'endof the time interval c-d, contact 30 opens the circuit before the resetting coil 22 is energized, which occurs at the. tirnea, to start a new cycle. Asa result of this arrangement, none of the relay contacts of the system conducts current at the moment of closing or opening, whereby the formation of. sparks is practically excluded and the lifetime of all the relays, as well as the reliability of the system, is greatly increased.

The rotation speed control circuit includes the secondary winding of the transformer 36, the primary winding 72 of a step-down transformer .74 and a plurality of relays, for example three, as shown in FIGURE 2 andadesignated by the numerals 76, 78 and 80. It will be understood that the number of switches will be chosen according to the service required, and it has been found that when using five switches, satisfactory results are obtained. Therelays 76, 78 and 80 are so connected that each of them,-when de-energized connects a determined number .of turns of the primary 72 into the rotation speed .control circuit. When energized, the corresponding number of turns is removed from the circuit. The relay 80, provided with two contact points 82 and 84, an armature86, and a coil 88, operates in such a manner that,

when the coil 88 is de-energized, the armature 86 connects'the contact 82 to the armature 90 of the following When energized, contact is made between the armature 8 6,and thecontact point 84 thus including only thenumber of-primary turns from the terminal 98 to the first tap 92 into the circuit. In an analogous manner, the relays 78 and 76 operate, when energized .to connect the primary turns from the terminal 98 to the taps 94 and 96, respectively, and to simultaneously disconnect the adjacent turns i.e., when energized, relay 76 disconnects the turns from the terminal 104 to the tap 96, relay 78 disconnects the turns from tap 96 to tap 94 and relay 80 disconnects the turns from tap 94 to tap 92. When all relays are tie-energized, the entire primary 72 is included in the circuit overall of the armatures 86, 98, 100 and the conductor 102. On the other hand when all relays are energized, only the turns from the tap 92 to the terminal 98 are connected in the circuit.

In order to energize the relays 76, 78 and 89 as a func tion of the rotation speed of the propeller 111), the propeller shaft 12 is provided with any conventional gear, for example a pair of spur gears 114 and 116 which drive a shaft 118. A number of switches, equal to the number of relays, are mounted on the shaft 113, the switches being responsive to the rotation speed of the shaft, prefera-.

bly of the kind customarily referred to as plugging switches. Centrifugal switches or generators producing currents depending on the rotation speed of the shaft 118 may also be used. In FIGURE 2, three such switches are represented by the numerals 120, 122 and 124 and the double-arrow in the symbol therefor represents the fact that each operates to close its contact126, 128 and 130 respectively when the propeller shaft 112 and con sequently the shaft 118 reaches a distinct rotation speed, regardless of the direction ofrotation. Assuming that switch 121) is responsive to a low rotation speed, switch 122 to a medium speed and switch 124 to a high speed, each switch energizes one of the coils 132, 134 and 88 of the relays 76, 78 and 88 respectively. At a rotation speed of zero or close to zero, neither of the plugging switches energizes the corresponding relay and the entire primary winding 72 is connected into the circuit. When the speed increases to the point where the plugging switch 120 is responsive, the coil 132 is energized over the closing contact 126 thereby disconnecting the conductor 102 and inserting the conductor 131 into the circuit. As a result, only the turns from the terminal 90 to the tap 96 remain in the circuit since the contacts 128 and 13llare still open, the relays 78 and 8d are still de-energized and the armatures 90 and 86 are still contacting the points 136 and 82, respectively. Similarly, at medium and high rotation speeds, the switches 122 and 124 cooperate with the relays 78 and St) to decrease the number of turns and thereby increase the current output from the secondary windings 149 of the step-down transformer 74.

The output circuit of the system includes the secondary windings 140 of the step-down transformer 74, one terminal of which is connected to the anode over the rectifier 142. From the other terminal 144 and over the resistors 146 and 148, respectively, line 1561 leads to hull and line 152 leads to the propeller 119 over the slip ring 154 mounted on the propeller shaft112 and the brush 156. As a result, the propeller and the hull 'are connected in parallel in the output circuit. Therefore, by adjusting the resistors 146 and 148, the current impressed on the propeller, on one hand, and on the hull, on the other hand, can be adjusted according to requirements, as described above.

As set forth hereinbefore, the propeller, requiring substantially more current than the hull, usually appropriate insulation of the propeller 119 and at least the current carrying portion of the shaft 112 will be necessary, this can be obtained by providing a shaft which is fabricated from two portions connected to each other by flanges 16%) and 162 having a disc of insulating material 164 mounted therebetween. The flanges 161' and 162 and the insulating disc 164 are firmly maintained in their position by means of bolts and nuts 166. The stern tube bearings, designated by the numeral 170, are generally water-lubricated and insulated by coatings 172 on all internal metal surfaces in contact with water, to ensure insulation from the hull.

When in operation, the described system performs in the following manner: The condition of the hull is monitored by the sensing circuit and the relay 12 adjusts the number of primary turns of the transformer 36 in the input circuit whereas the speed control circuit regulates the power supply, as received from the input circuit, in accordance with the rotation speed of the propeller. Finally, the ratio between the values of the resistors 146 and 148 determines the fraction of the entire output which is supplied to each, to the hull and to the propeller. In order to adjust the system according to the prevailing conditions, the variable resistor 8 is set so that no current flows through the sensing circuit when the hull potential is at the predetermined optimum. The tap leading to the contact point 54 is selected in such a way that the power input induced to the speed control circuit by the secondary 79 will result in a minimum output supplied to the hull, which is further reduced by the resistor 146. This is the case even when the propeller is rotating at a maximum speed. Then the current impressed on the propeller will be still sufiicient to afford protection against cavitation whereas the paint of the hull is simultaneously protected against damage. It has been established that the ratio between the potential impressed on the propeller and the potential impressed on the hull suitably ranges between the value 5:1 and 15:1, this value, preferably 9:1, is the ratio between the values of the resistors 146 and 148 and represents an average optimum because the unpainted propeller requires much more current than the painted hull and, as pointed out already, the comparatively strong current supplied to the propeller result in stray currents to the hull which in practice hardly can be avoided even so, when providing insulation of the current carrying propeller shaft portion, as described. It will be apparent, that the sensing components of the system, i.e., the reference half cell in cooperation with the components of the sensing circuit register every change of the hull potential, which includes changes of the hull potential due to stray currents produced by the system itself. Therefore, the sensing components perform simultaneously as a safety device to prevent damage to the paint film on the hull.

Many modifications of the cathodic protection system according to the invention are possible. The resulting differential current in the sensing circuit can be utilized, for example, for biasing the grid potential of a suitable electron tube, which registers the direction of the current and thereby controls the power in the output circuit.

Furthermore, only one power supply 38 is shown in the drawings, for the purpose of simplification, whereas when the system is actually installed on a ship, the energizing circuits for the relays and switches are fed from a low current source or the output of transformers. The plugging switches 12%, 122 and 124 can bereplaced by centrifugal switches or, as another equivalent which performs to produce the same result, generators may be mounted on the shaft 113 or driven otherwise by the propeller shaft 112; the output of these generators is then lead to switches responsive to a distinct electromotive force, which switches will operate to energize the coils 126, 128 and as a function of the speed of rotation of the propeller.

It has been pointed out already that the cathodic protection system, as described, is illustrated herein schematically and as fed by a single-phase power supply for clarity and simplification. in the systems actually installed on ships, the power supplied to the input circuit is preferably a three-phase alternating current. Whereas the sensing circuit remains unchanged, the transformer 36 is actually a three-phase transformer of which the primaries are connected as a three-phase six-wire system and in such a way that the connections are made either in a A-system or in a Y-system. The snap action double throw-switch 50'is replaced by a pair of, suitablyinter "locked;- relays which operate to switch, from the A-system to the Y-system and back,'according to the. electrical signal emitted by the sensing'relay'. This relationship is illustrated in FIGURE 1 by showing that the power 1s transmitted from-the input circuit to the rotation speed control circuit by a three-phase transformer connectedas t e a A-system or a Y-system, whereby theY-system corresponds to the-position of the armature 4S connecting the contact point 52, thus constitutingthelow output position, whereas the A-connection corresponds to the position of the armature 48 connecting thecontact point 54, which is the high output position.

As defined above, the cathodic protection system has been described and is shown in the drawingin'connection with one propeller and one anode in the output circuit. When installed on a ship, it is assumed that all propellers rotate at'substantially the same speed .so'that' only one system of, generally five, rotation speed respon- 'sive switches is used. However, each relay operating in accordance with one plugging switch operates to insert or to remove a distinct number of turns in each primary 7*2 pertaining to one-phase of the transformer 74. The. outputsof all threeisecondaries are combined and rectified to'result in a power output which is an only very slightly pulsating direct current. Actually, a plurality of anodesand reference cells are'usedand the anodes are arranged on thelhull at the distances from each other required: in

the powersupply being connected to the propeller and to the hull in parallel, adjustable means for varying the ratio 'between the power supplied to the hull and the power supplied to the propeller; and means for varying the amount of power supplied to the terminals as a direct 7 function. of the rotation speed of the propeller..

accordance with the generalco'n'dition to ensure the cor-1 rect currentdensity for each'portion of the hull. anodes which cooperate to supply the propellers with protecting current are mounted at the stern POItlOH'OfT the hull and as close as possible to and facing the propel lers. It has been found advantageous to expand the customary, insulatingblanket affixed to the'hull so that most of the stern portion is insulated, in order to prevent the flow of excess current from the anodes to'the hull instead of tothe propeller.

. It willbe obvious to those skilled in the art that many modifications may be made within the scope of the present inventionwithout departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

' 1. A system for cathodically protecting the propeller of a shipfioating in water which comprises a'power'supply f having its negagtive terminal connected to the propeller,

adjusting the power supplied to the propeller as a direct function of the rotation speed of "the propeller. v

2.; A system for cathodically protecting the propeller of a ship floating in water which'comprises a power supply connected to the propeller, an anode connected to the power supply and immersed in the water and a plurality of'switching means, each of which-is responsive to one distinct rotation speed of the propeller and adapted to increase the power supplied to the anode above the power supplied at a lower rotation speed;

3. A system for cathodically protecting the propeller of l a ship'floating in water comprising a rotationspeed control circuit and an output circuit, the rotation speed control circuit including a power supply, the tapped primary windingof a transformer and aplurality of relays,

each relay being adapted to be energized by .a switch responsive to a distinct rotation speed of the propeller and 7 adapted to'vary the number of; energized primary wind- 7 are mounted on'a-common shaft which is mechanically coupled to the propeller, shaft, eachswitch being inserted in the energizing circuit of one of the relays.

7. A system for cathodically protecting thehull and the propeller of a ship floating in water comprising a power supply, an anode immersed in the water and connected to one terminal of the power supply, the other terminal of the power supply being connected to the propeller and to the hull in parallel and means for varying the power supplied to the terminals as a function of the hull potential and as a direct function of the rotation speed of the propeller.

8. A system for cathodically protecting thehull and the propeller of a'shi-p floating in water comprising a power supply, a sensing circuit, an input circuit, a speed control circuit and an output circuit; the sensing circuit including the hull, a referencehalf cell, a source of electromotive force opposedto and adjusted to reduce to zero the electromo'tive. force generated by the reference half cell when the hull potential reaches a predetermined opti mum and means responsive t'o-the'direction' of theresult-- ing differential current; the inputcircuit including the power supply, the primary winding of a first transformer and means for adjusting the power supplied to the primary winding of the first transformer asa function of the direction of the resulting difierential current. in the sensing circuit; the speed control circuit including the secondarywinding of the first transformer, the tapped primary winding of a second'transformer 'and means for adjusting the power supplied to the primary winding of the second an anode connected to the positive powersupply terminal. and immersed in the water, and means for automatically transformer as a direct function of the rotation speed; theoutput circuit comprising, in series, an. anode imnrersed in thewater, a rectifier, the secondary winding of the second transformer andtwo conductors in parallel,

the first conductor being connected tothe propeller and' being a three-phase transformer adapted to be connected as a Y-system when the electrornotive force generated by the reference'half cell exceeds the opposed electromotive force and adapted to be connected as a A-system when the electroniotive force of the opposed source exceeds that generated'by the reference half cell.

10. A system according to claim 9 comprising interlocked relays to operate the switching between the Y-sysl sensing circuit.

4. A system according to claim 3 wherein the switches- 11. A'system according to claim 8 in which the sensing circuit includes a resistor connected in parallel with the source of opposedelectromotive force;

12. A system according to claim 8 in which the means responsiveto'the direction of the resulting difierential current in the sensing circuit is a relay.,

7 l3. system according to claim 12 in which the sensing relay has a first and a second contact, the relay armature cooperating with the first contact when the electromotive force generated by the reference half cell exceeds the electromotive' force of the opposed source, and cooperating with the second contact whenthe opposed electrornotive force exceeds that generated by the reference .half cell; the first contact being adapted to energize means for decreasing the power. supplied tothe speed control circuit and the second contact being adapted'to 9 energize means for increasing the power supplied to the speed control circuit.

14. A system according to claim 12 in which the sensing relay is provided with an electrical resetting device which is periodically and intermittently energized by a time switch.

15. A system according to claim 14 comprising a time switch having a first and a second periodically operated pair of contacts, the first pair of contacts adapted to energize peridoically and intermittently the resetting device of the sensing relay, the second pair of contacts being adapted to connect the power supply to the input circuit, the second pair of contacts being operated periodically and within the de-energized intervals of the first pair of contacts.

16. A method for cathodically protecting the propeller of a ship floating in water which comprises generating an electromotive force, supplying the electromotive force to an anode immersed in the Water and to the propeller as the cathode to produce a potential diiference therebetween and adjusting the electromotive force as a direct function of the rotation speed of the propeller.

17. A method according to claim 16 which comprises supplying the electromotive force to the anode and to the propeller as a substantially linear function of the rotation speed to produce a current density of about 25 to 75 milliamperes per square foot of propeller surface when the rotation speed is about zero and of about 250 to 350 milliamperes per square foot of propeller surface when the rotation speed is about 250 to 350 revolutions per minute.

18. A method for cathodically protecting the propeller of a ship floating in water which comprises generating an electromotive force, supplying the electromotive force to an anode immersed in the water and to the propeller as the cathode to produce a potential diflerence therebetween, registering the rotation speed of the propeller in terms of an electromotive force and adjusting the electromotive force supplied to the anode and to the propeller as the cathode as a direct function of the registered electromotive force.

19. A method for cathodically protecting the propeller and the hull of a ship floating in water which comprises generating a first electromotive force, supplying the first electromotive force to an anode immersed in the water and to the propeller and to the hull as the cathodes, registering the rotation speed of the propeller in terms of a second electromotive force and adjusting the first electromotive force as a direct function of the second, registered electromotive force.

20. A method for cathodically protecting the propeller and the hull of a ship floating in water which comprises generating a first electromotive force, supplying the first electromotive force to an anode immersed in the water and to the propeller and to the hull as the cathodes, and registering the rotation speed of the propeller in terms of a second electromotive force; generating a third electromotive force between a reference half cell and the hull, opposing the third generated electromotive force with a constant electromotive force such that the resulting electromotive force is zero when the potential at the hull reaches a predetermined optimum, and registering the direction of the resulting difierential electromotive force; and adjusting the first electromotive force according to the registered direction and as a direct function of the second, registered electromotive force.

21. In an impressed current cathodic protection system, a metallic object in contact with an electrolyte and a power source for applying a current through the electro- 19 lyte to said object as a function of two varying conditions, means for adjusting the current according to the first condition, and means for modifying the adjusted current as a direct function of the second condition.

22. In an impressed current cathodic protection system, a metallic surface in contact with an electrolyte in a moving relationship at a varying speed, a power source for applying a current through the electrolyte to said surface, means for adjusting the current according to polarization conditions on the metallic surface, and means for modifying the adjusted current as a direct function of said speed.

23. In an impressed current cathodic protection system, a metallic surface in contact with an electrolyte in a moving relationship at a varying speed, a power source for applying a current through the electrolyte to said surface as a function of polarization conditions on said surface, first sensing means for producing a first signal according to said polarization conditions, means responsive to said first signal for adjusting said current, second sensing means responsive to said speed for producing a second signal as a direct function of said speed, and means for modifying the adjusted current as a direct function of said second signal.

24. In an impressed current cathodic protection system for preventing the corrosion of the hull and of the propeller of a ship, a power source for applying a protective current through the water to the hull and to the propeller, a reference half-cell responsive to polarization conditions on the hull, means for adjusting said protective current as a direct function of the electromotive force generated by said reference half-cell, and means responsive to the rotation speed of the propeller for modifying the adjusted current as a direct function of said rotation speed.

25. In an impressed current cathodic protection system for preventing the corrosion of the propeller of a ship, a power supply having its negative terminal connected to the propeller shaft and to the hull of the ship, the current carrying portion of the propeller shaft having a first flange mounted thereon, which flange is secured to a second flange mounted on the remaining portion of the shaft, a disc of insulating material maintained between the first flange and the second flange, and means for providing higher current density on the propeller surface than on said hull.

26. The cathodic protection system according to claim 25, in which said power supply has its negative terminal connected to said propeller shaft, the shaft being supported by water lubricated bearings, the internal surfaces of the hearings which are in contact with the water having an insulating coating thereon.

27. In an impressed current cathodic protection system, a surface for contacting an electrolyte, an anode mounted adjacent said surface, means for applying current between said anode and said surface, means for moving said surface relative to said electrolyte at different speeds, and means directly responsive to the rate of relative movement for varying the level of current applied between said anode and said surface.

References Cited in the file of this patent UNITED STATES PATENTS 921,641 Cumberland May 11, 1909 2,402,494 Hantzsch et a1. June 18, 1946 2,759,887 Miles Aug. 21, 1956 FOREIGN PATENTS 503,946 Great Britain 1939 

16. A METHOD FOR CATHODICALLY PROTECTING THE PROPELLER OF A SHIP FLOATING IN WATER WHICH COMPRISES GENERATING AN ELECTROMOTIVE FORCE, SUPPLYING THE ELECTROMOTIVE FORCE TO AN ANODE IMMERSED IN THE WATER AND TO THE PROPELLER AS THE CATHODE TO PRODUCE A POTENTIAL DIFFERENCE THEREBEWEEN AND ADJUSTING THE ELECTROMOTIVE FORCE AS A DIRECT FUNCTION OF THE ROTATION SPEED OF THE PROPELLER.
 21. IN AN IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM, A METALLIC OBJECT IN CONTACT WITH AN ELECTROLYTE AND A POWER SOURCE FOR APPLYING A CURRENT THROUGH THE ELECTROLYTE TO SAID OBJECT AS A FUNCTION OT TWO VARYING CONDITIONS, MEANS FOR ADJUSTING THE CURRENT ACCORDING TO THE FIRST CONDITION, AND MEANS FOR MODIFYING THE ADJUSTED CURRENT AS A DIRECT FUNCTION OF THE SECOND CONDITION. 